Many similarities exist between neural plasticity during development and the plasticity associated with learning and memory in the adult. The mollusc Aplysia provides an excellent model system for studying the mechanisms of development and learning in parallel. Recent work has revealed that neurotrophic factors, traditionally studied with respect to developmental plasticity, may play a role in adult organisms during learning and memory. The goal of the proposed research is to study the role of neurotrophic factors in Aplysia in the process of long-term synaptic facilitation of the neurons underlying defensive withdrawal reflexes. Using the Aplysia model it will be possible to determine when and how neurotrophic factors work in modulating synaptic response and then compare these forms of modulation with the actions of growth factors in development in the same experimental system.